.\"
.\" Copyright (c) Mark J. Kilgard, 1996.
.\"
.TH glutEstablishOverlay 3GLUT "3.7" "GLUT" "GLUT"
.SH NAME
glutEstablishOverlay - establishes an overlay (if possible) for the
current window. 
.SH SYNTAX
.nf
.LP
void glutEstablishOverlay(void);
.fi
.SH DESCRIPTION
glutEstablishOverlay establishes an overlay (if possible) for the
current window. The requested display mode for the overlay is
determined by the initial display mode.
glutLayerGet(GLUT_OVERLAY_POSSIBLE) can be called to
determine if an overlay is possible for the current window with the
current initial display mode. Do not attempt to establish an overlay when
one is not possible; GLUT will terminate the program. 

If glutEstablishOverlay is called when an overlay already exists,
the existing overlay is first removed, and then a new overlay is
established. The state of the old overlay's OpenGL context is discarded. 

The initial display state of an overlay is shown, however the overlay is
only actually shown if the overlay's window is shown. 

Implicitly, the window's layer in use changes to the overlay immediately
after the overlay is established. 
.SH EXAMPLE
Establishing an overlay is a bit involved, but easy once you get the
hang of it.  Here is an example:
.nf
.LP
  int overlaySupport;
  int transparent, red, white;

  glutInitDisplayMode(GLUT_SINGLE | GLUT_INDEX);
  overlaySupport = glutLayerGet(GLUT_OVERLAY_POSSIBLE);
  if (overlaySupport) {
    glutEstablishOverlay();
    glutHideOverlay();
    transparent = glutLayerGet(GLUT_TRANSPARENT_INDEX);
    glClearIndex(transparent);
    red = (transparent + 1) % glutGet(GLUT_WINDOW_COLORMAP_SIZE);
    white = (transparent + 2) % glutGet(GLUT_WINDOW_COLORMAP_SIZE);
    glutSetColor(red, 1.0, 0.0, 0.0);  /* Red. */
    glutSetColor(white, 1.0, 1.0, 1.0);  /* White. */
    glutOverlayDisplayFunc(redrawOverlay);
    glutReshapeFunc(reshape);
  } else {
    printf("Sorry, no nifty overlay (try an SGI workstation)!\n");
  }
.fi
.LP
If you setup an overlay and you install a reshape callback, you need
to update the viewports and possibly projection matrices of both the
normal plane and the overlay.  For example, your reshape callback
might look like this:
.nf
.LP
  void
  reshape(int w, int h)
  {
    if (overlaySupport) {
      glutUseLayer(GLUT_OVERLAY);
      /* Setup overlay to have X style coordinate system. */
      glViewport(0, 0, w, h);
      glMatrixMode(GL_PROJECTION);
      glLoadIdentity();
      gluOrtho2D(0, w, 0, h);
      glScalef(1, -1, 1);
      glTranslatef(0, -h, 0);
      glMatrixMode(GL_MODELVIEW);
      glutUseLayer(GLUT_NORMAL);
    }
    glViewport(0, 0, w, h);
  }
.fi
.LP
See the glutOverlayDisplayFunc man page for an example showing one way to write
your overlay display callback.
.SH X IMPLEMENTATION NOTES
GLUT for X uses the SERVER_OVERLAY_VISUALS convention is
used to determine if overlay visuals are available. While the convention
allows for opaque overlays (no transparency) and overlays with the
transparency specified as a bitmask, GLUT overlay management only
provides access to transparent pixel overlays. 

Until RGBA overlays are better understood, GLUT only supports color
index overlays. 
.SH SEE ALSO
glutUseLayer, glutRemoveLayer, glutCreateWindow, glutPostOverlayRedisplay, glutShowOverlay,
glutOverlayDisplayFunc
.SH AUTHOR
Mark J. Kilgard (mjk@nvidia.com)
